Delay line for travelling wave tube



April 27, 1965 E. A. ASH 3,181,090

DELAY LINE FQR TRAVELLING WAVE TUBE Filed 001;. 27. 1960 4 Sheets-Sheet1 H61. FIG. F1 6;. v v 4 Inventor JJSH April 27, 1965 E. A. ASH3,181,090

DELAY LINE FOR TRAVELLING WAVE TUBE Filed Oct. 27, 1960 4 Sheets-Sheet 2FEGES.

l I400 I600 [800 'ZOOO 2200 I n venlor A Home y April 27, 1965 E. A. ASH3,181,090

DELAY LINE FOR TRAVELLING WAVE TUBE Filed Oct. 2'7, 1960 4 Sheets-Sheet5 Inventor E-A-ASH By P Attorney April 21, 1965 E, A, ASH 3,181,090

I DELAY LINE FOR TRAVELLING WAVE TUBE Filed Oct. 27. 1960 4 Sheets-Sheet4 /7 mm 3 we A Home y United States Patent 3,181,090 DELAY LINE FORTRAVELLING WAVE TUBE Eric Albert Ash, London, England, assignor toInternational Standard Electric Corporation, New York, N.Y. Filed Oct.27, 1960, Ser. No. 65,304 Claims priority, application Great Britain,Nov. 30, 1959, 40,545/59 7 Claims. (Cl. 333-31) The present inventionrelates to slow Wave structures such as are incorporated in travellingwave tubes and backward wave oscillators.

The conventional helix slow wave structure has many known disadvantagesfor tubes of the highest frequencies, particularly from the point ofview of manufacturing difificulties. An alternative slow wave structure,or" which there are several variants, consists, basically, in aladderlike arrangement of half wave resonators, in the form of bars orwires, stretched between parallel plane conducting side walls. Aperfectly symmetrical arrangement of this type does not propagate over afinite band of frequencies, but may be made to propagate by introducingsome asymmetry in the geometrical arrangement which will make themagnetic and electric couplings between adjacent resonators unequal. Thepresent invention may be regarded, in one aspect, from the viewpoint ofthis asymmetrical class of slow wave structure. According to the presentinvention there is provided a slow wave structure which will propagateradio frequency waves over a frequency band whose width exceeds fourpercent of the mid-band frequency comprising a pair of parallel planeconducting walls and a grating joining the two side Walls along thelength of the structure in a plane perpendicular to the side walls withthe side walls extending above and below the plane of the grating, andin which the grating is composed of intersecting conducting strandsarranged to form a regular pattern of staggered apertures.

Many possible patterns of grating may be used, some more advantageouslythan others from the point of view of either mechanical construction orof bandwidth. In general it may be taken that for travelling waveampliher or backward wave oscillator use, a frequency band less than 4%of the midband frequency is of little value. At present we prefer one oftwo general types of grating pattern: in the first, the pattern is ofdiamonds, produced by the intersection of two oppositely inclined setsof equidistant parallel strands, with, as a special case, what may becalled a half diamond pattern, where the oppositely inclined strandsintersect only at the side walls.

The other preferred pattern, which may be regarded as derived from ameander line, has a zig-zag conducting path extending from end to end ofthe slow wave structure in between the side walls and supporting strandsjoin this meander line at regular intervals to the side walls.

A variant of the meander line structure is produced by a set ofequidistant parallel strands stretched from side wall to side wall atright angles to them and shorter strands, parallel to the side walls,each joining an adjacent pair of the transverse strands and off-set fromone another so as to form a pattern of apertures similar to theconventional arrangements of bricks in a wall, there being normally afraction of a whole brick terminating each row. This last mentionedvariant can be regarded as a version of the diamond pattern in which thediamonds are distorted into rectangles.

The slow wave structures of the invention have mechanical and electricalcharacteristics which render them particularly suitable for use eitherfor comparatively low frequency, high power, tubes or for millimetrewave tubes.

"ice

Unlike the helix, no insulation is required between the slow wavestructure and surrounding parts. The dispersion characteristics are suchas would be suitable for a slow wave structure used in conjunction witha high voltage electron beam, while the strands of a structure can bemade in the form of hollow pipes so that it is eminently suitable foruse in a high power low frequency travelling wave tube. On the otherhand, small gratings can readily be produced by photo-etchingtechniques, and the structures exhibit strong spatial harmoniccomponents in their propagation characteristics; they are thus verysuitable for use as slow wave structures at the shortest wavelengths,for example in a backward wave oscillator at millimetre wavelengths.

The invention will be further described with reference to theaccompanying diagrammatic drawings in which:

FIG. 1 represents a perspective view of one end of a slow wave structureaccording to the invention;

FiGS. 2 to 5 illustrate plan views of the grating of PEG. 1, eachshowing a different diamond pattern of apertures;

FIG. 6 shows, qualitatively, dispersion characteristics ofdiamond-apertured structures of the invention;

FIGURE 7 illustrates a meander line pattern of grating according to theinvention;

FIG. 8 illustrates a modification of the arrangement of FIG. 7 suitablefor water cooling;

FIG. 9 illustrates a brick wall variant of the meander line structure ofFIG. 7;

FIGS. 10 to 12 illustrate respective variants of the arrangement'of FIG.9; I

FIGS. 13 and 14 show measured dispersion characteristics of variouslydimensioned diamond and meander line structures respectively illustratedin FIGS. 13a and 14a; 7

FIG. 15 illustrates a means of coupling a slow wave structure accordingto the invention to a hollow rectangular waveguide; and

PEG. 16 illustrates an embodiment of a slow wave structure according tothe invention in a travelling wave tube.

In all its variants the slow wave structure of the present inventioncomprises, as indicated in FIG. 1, a pair of plane parallel side walls 1and 2, indicated as spaced apart a distance a, with a conducting grating3 joining the walls in a plane perpendicular to them and running thelength of the structure. The strands of the grating are arranged toproduce an array of intersecting conductors forming a regular pattern ofstaggered apertures. A typical type of pattern is indicated in FIGS. 2to 5, in all of which there are two sets of equidistant parallel strandsstretching from side wall to side wall, the two sets making equal andopposite angles with the normals to the side walls, and in which, exceptfor the extreme end strands of the pattern, each strand is joined at itsends to respective strands of the other set.

Mention has been made above of the resonator type of slow wave structurein which an array of similar half wave resonators in stretched between apair of parallel side walls. By inclining the resonators to the walls anasymmetry in coupling of the electric and magnetic fields of adjacentresonators is produced and propagation can occur over a limitedbandwith. A bandwidth of such or Hartree harmonics, whilethat-illustrated in FIG. 2

has also a fundamental backward mode which the contrawound helix doesnot possess. The .side walls are essential constituents of the slow wavestructure and determine the low frequency cut-off of the propagationcharacteristics.

In the pattern of FIG. 2 one set of equidistant parallel strands 4stretches from side wall to side wall and is joined, at its ends byrespective strands of a similar set, strands 4 and 5 being oppositelyinclined to the walls. In FIG. 2 the strands of the two sets intersectonly at the side Walls and so produce a pattern or triangles which,compared with the other patterns of FIGS. 3 to 5, can be regarded asbeing a pattern of half diamonds, as indicated by the symbolic legend tothe rightof the figm'e.

In FIG. 3 the strands 4 and 5 of the respective two sets intersect oneanother once intermediate the side walls so as to produce fulldiamond-shaped apertures between the walls. This pattern we refer to asa single diamond pattern, a complete diamond reaching from side wall toside wall, as represented by the single diamond symbol to the right ofthe figure. 7

' With each strand of one set intersected twice by strands of the otherset, we obtain 'the one and a' half diamond pattern of FIG..4, whilewith three intersections of each strand the two diamond pattern of FIG.Sis obtained.

It is possible, by increasing the number of intersections, to producefurther diamond patterns in whichany number of complete diamonds or ofcomplete diamonds plus a half diamond, joined end to end, fit in thetransverse dimension of the slow wave structure. As the number ofintersections is increased, however, the thickness of the strandsreduces the open area of the grating occupied by apertures. i

The dispersion characteristics of the diamond structures of the presentinvention are illustrated qualitatively in the Brillouin plot of FIG. 6.Here the angular frequency a of the propagated waves is plotted asordinate against the phase constant e as abscissa. namely those typicalof the half-diamond, one-diamond, and twodiamond patterns respectively.All have the same lower cut-off frequency, denoted by the ordinate1rC/a, where c is the velocity of light and a is the separation betweenthe conducting planes 1 and 2 of FIG. 1. At this cut-off frequency thephase velocity is equal to the velocity of light, so that the curvesintersect the straight line v=c passing through the origin as indicatedin FIG. '6, v

being the velocity of propagation along the line. In the Brillouin plotthe velocity of propagation v=w/;8 while the group velocity is equal to7 the slope of the curve. For the half-diamond structure thedispersioncurve of the Brillouin plot is of low slope and ischaracterised by a branch, shown dotted, in which the slope isnegativei.e. the group velocity is oppositely directed to the phasevelocity; thus the half-diamond structure exhibits a fundamentalbackward mode of propagation. The other patterns of FIGS. 3 to 5 do nothave a fundamental backward mode of propagatiombut all exhibit strongfirst spatial harmonic modes. For these harmonic modes thephase'constant ,8 is simply related to the phase constant in thefundamental mode,'so that the Brillouin plots are generally similar tothose for the fundamental modes. As the number of diamonds across thepattern is increased; so there is an increase in the upperfrequencycut-oif of the slow wave structure- As the pitch p betweenadjacent wires (FIG. 2) is increased,

Three curves are shown,

in any of the patterns, the velocity of propagation is increased, thevelocity increase being nearly proportional to the increase in pitch(except near the cut-off frequencies).

The characteristic impedance of the slow wave structures is high, and,although the bandwidth of, for example, the half-diamond structure, is.somewhat:low, the product of bandwidth and impedance of the circuitturns out to be nearthe optimum for use in a travelling wave amplifier.

Mention has been made above of the mechanical difficulty which may beencountered when an attempt is made to increase the number of diamondsacross the width of the slow wave structure. This may be overcome, to alarge extent, by modifying the pattern so as to obtain, for example, anequivalent arrangement of rectangles, such as illustrated in FIG. 9.These modified structures can also be regarded as derived from a meanderline stretching from end to end of the slow wave structure. In FIG.

7 there is shown a pattern in which a rectangular zig-zag continuousconductor 6 stretches from end to end of the structure midway betweenthe sidewalls 7 and-8. The conductor 6 is supported at each bend by aconducting stub 9 joining. the meander line to the respective adjacentside walls of the structure.

The arrangement of FIG. 7 can be compared to the half-diamond structureof FIG. 2, and its propagation characteristics are generally similar.

There is no need for the meander line to be of strictly rectangularshape; the bends may be rounded and, as is illustrated verydiagrammatically in FIG. 8, the grating may be made of hollow pipingrather than fine strands. In FIG. 8 a sinous piping 10 replaces themeander line 6 and the struts 9 are replaced by further pipes 11 whichjoin the piping 10 to hollow side walls-12 and 13. Cooling liquid ispumped into the side wall 12 and flows through the piping network to theopposite hollow wall 13.

The pattern of FIG. 7 may be modified by arranging that the side strutsextend from wall to wall. We then obtain a pattern such as that of FIG.9. In FIG. 9 the struts 9 become portions of a set of parallelequidistant strands 14 stretchingnormally between the'side walls, andaset of short strands 15 is arranged parallel to the side walls with eachstrand 15 joining two adjacent strands of the set 14, adjacent strands15 being staggered sothat the pattern of apertures resembles that of theconventional briclobuilt wall, each row containing one and a halfbricks. In the drawing the strands 15 and the portion of the strands 14between them has been thickened to show the correspondence with themeander line 6 of FIG. 7. On the other hand, comparison with FIG. 4shows that the pattern'of FIG. 9 can be regarded as a modified one and ahalf diamond structure, to which it has similar propagationcharacteristics. More than one strand 15 can be placed between adjacentstrands 14, as for example is illustrated in the FIG. 10, which wouldcorrespond to a two and a half diamond structure. .Alternatively thepattern of FIG. 10 can be regarded as a double meander line structure inwhich two meander lines such as 6 of FIG. 7 can be traced out side byside.

In arrangements otherwisesimilar to those of FIGS. 7 to 10 it is notessential that the grating pattern be orthogonal to the side walls. Thusin FIG. 11 the strands 14 are inclined to the side walls, the strands 15remaining parallel to the side walls, so that we now have a staggeredpattern of parallelograms corresponding to the one and a half diamondpattern of FIG. 4. In the arrangement of FIG. 12 the strands 15 havealso been inclined to the side walls, so that there is no longer acomplete analogy with the patterns of FIGS. 2 to 5. The propagationcharacteristics. of an arrangement such as that of FIG. 12 will,however, not be altogether dissimilar to those of the other structuresso far described, although the bandwidth will be considerably less thanthat of the straightforward diamond structures.

Measured dispersion characteristics of several diamond and meander linestructures according to the invention are shown in FIGS. 13 and 14respectively, the size of structure and the frequency range having beenchosen purely from the point of view ofease of measurement. In FIG. 13the ratio v/c of phase velocity to the velocity of light is plottedagainst frequency in megacycles per second. The curves lettered A to Ccorrespond to the similarly lettered structures illustrateddiagrammatically in FIG. 13a. In each case the spacing between the sidewalls, indicated by a in FIG. 13a, was 12 cm. The distance betweenadjacent wires is indicated in FIG. 13a by the dimension p, /217, or 2pas the case may be; p=1 cm. in all cases.

The curves of FIG. 13 show, as has been discussed previously inconnection with FIG. 6, how the bandwidth increases with an increasingnumber of intersections of the strands and also shows, in curves A and Band in E and F, the somewhat slight increase in phase velocity obtainedwhen the pitch is increased. Thus curves E and F both relate to aone-and-a-half diamond structure but in curve E the pitch is /2 cm. andin curve F 1 cm.

In FIG. 14 the curves A, B and C relate to structures arranged asindicated schematically in FIG. 14a. In all these the pitch p was 1.7cms. and the separation between sidewalls 12 cms.; in A and B thedimension d was 6 ems. and b 3 cms., but in C b and d were each 4 cms.Structure A has analogies to the half-diamond structure of FIG. 13, Bcan be regarded as a modified one-and-aquarter diamond structure, and Cas a modification of the one-and-a-half diamond structure.

In connection with the curves of FIGS. 13 and 14 and the experimentstructures to which they relate, it is to be emphasized that theabsolute values of the frequencies plotted and of the dimensions of thestructures given above should not be taken as illustrative of anypreferred embodiments of the invention; the frequencies and dimensionswere chosen purely from the point of view of convenience in making themeasurements involved and in setting up different patterns of grating.Useful design data may, however, be derived from these curves byconsidering the variation of v/c in terms of the ratio of the frequencyof measurement to the lower cut-off frequency, f,,, which, from FIG. 6,is seen to be given by and by using, instead of the pitch parameter p, ageneralised parameter obtained by expressing p as a portion of the freespace wavelength.

Slow wave structures according to the present invention are very readilycoupled to hollow rectangular waveguides. Basically, all that isnecessary is to continue the grating a little way beyond the ends of theside walls so that it projects through a slot cut transversely in thebroad side of the waveguide. Thus in FIG. 15 a hollow rectangularwaveguide is represented at 16 having a transverse slot 17 cut in abroad side 18. The side walls 1 and 2 of a slow wave structure accordingto the invention are brought up to the wall 18 so that an extension 19of the grating 3 projects through the slot 17 about half way into thewaveguide. The lips of the slot 17 are provided with curved fins 20which help to provide a smooth impedance transformation between thewaveguide and slow wave structure and also act as chokes preventingradiation through the slot 20.

FIG. 16 illustrates the use of a slow wave structure according to theinvention in a travelling wave amplifier tube. Since there is no needfor any insulation in association with the slow wave structure the tubemay be of all metal construction having a rectangular envelope 20 whosesides 21 function as the side walls of the slow wave structure with thegrating 3 joined between them. The upper and lower walls should be awavelength or more spaced from the grating 3 so as not to interfere withthe performance of the slow wave structure. At each end of the slow wavestructure a waveguide 16 is arranged as in FIG. 15 to serve as input andoutput feeds respectively. The waveguides are represented as havingtuning pistons 22 for adjusting the match between slow wave structureand waveguide and are indicated as being hermetically sealed by means ofwaveguide windows 23. The waveguides 16 are apertured for passage of anelectron beam above and below the grating 3 and are secured toextensions 24 and 25, respectively, of the envelope 20. An electron gun26 is indicated within the end enclosure 24 and an electron collectorelectrode 27 is represented within the extension 25.

In the case of applicationof the invention to a backward waveoscillator, the arrangement is generally similar to that of FIG.v 16except that the waveguide adjacent the electron collector electrode isomitted and the end portion of the grating 3 adjacent the electroncollector electrode is coated with lossy material to provide arefiectionless termination for the slow wave structure.

Although several embodiments have been illustrated in the accompanyingdrawings, numerous other variants of the invention will occur to thoseskilled in the art. Although the propagation constants will, in allcases, conform to the general characteristics as described withreference to FIG. 6 and to FIGS. 13 and 14, it does not follow that allstructures whose geometry appears to :fall within the generalgeometrical arrangement of embodiments of the invention will necessarilyhave useful band- Widths; each structure would need to be examinedindividually to determine the exact shape of the characteristic curvesand to determine whether the bandwidth available, at the fundamentalmode, falls within the four percent limit which we consider to berequisite for useful application.

It may be mentioned that an experimental backward Wave oscillator hasbeen made using a structure similar to C, FIG. 14a, scaled down,however, to the 3 cm. band thus the 12 centimetre width betweensidewalls was reduced to 15 millimetres and the pitch between wiresreduced accordingly. Oscillations on the first backward mode wereobtained, the voltage and frequency characteristics being in goodagreement with the figures predicted from cold tests.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

What I claim is:

1. A slow wave structure which will propagate radio frequency waves overa frequency band whose width exceeds four percent of the mid-bandfrequency comprising a pair of parallel plane conducting side walls anda grating connecting the two side walls along the length of thestructure in a common plane perpendicular to the side walls with theside walls extending above and below the plane of the grating, and inwhich the grating is composed of continuous intersecting conductingstrands arranged to form a regular pattern of staggered apertures, saidgrating being operablerin both amplifier and backward wave tubes.

2. A slow wave structure according to claim 1 in which the strands ofthe first set are each normal to the side walls and the strands of thesecond set are perpendicular to those of the first set and arranged toform a brick-like pattern with a fraction of a brick terminating eachrow.

3. A slow wave structure which will propagate radio frequency waves overa frequency band whose width exceeds four percent comprising a pair ofparallel plane conducting side walls and a grating connecting the sidewalls along the length of the structure in a common plane perpendicularto the side walls with the side walls extending above and below theplane of the grating in which 7 the grating is composed of a'first setof equidistant continuous intersecting conducting strands stretchingobliquely from sidewall to side walland a second set of conductingstrands similarly arranged at equal and opposite anglesofinclinationtothe side walls, each strand of one set intermediate theend strands of the grating being joined at both ends tothe ends ofrespective strands of the other set, said grating being operable in both7 amplifier and backward tubes.

4. A slow wave structure which will propagate radio frequency waves overa frequency band whose width exceeds four percentof the mid bandfrequency comprising a pair of parallel plane conducting side walls anda "grating connecting the two side walls alongthe length of thestructure in a common plane perpendicular to the side walls with theside walls extending above and below the plane. of the grating, and inwhich the grating is composed of a-regular continuous zig-zag conductingstrand centrally disposed from end to end of the grating and an 6. Aslow wave structure according to claim 1 in which the grating iscomposed of a first set of equidistant conducting strands stretchingfrom side wall to side wall and a second set of shorter conductingstrands each joining a respective one pair only of the strands of thefirst set intermediate the ends of the pair.

7. A slow wave structure according to claim 3 in which the strands ofthe two sets intersect each other only at their ends.

10 References Cited by the Examiner UNITED STATES PATENTS 2,708,236 5/55Pierce 333-31 X 2,823,332 2/58 'Fletcher 315-3.5 15 2,831,142 4/58 Kazan3153.6 2,834,915 5/58 Dench 33331 X 2,882,440 4/59 Mourier 333-31 X2,890,384 6/59 Dench 3333l X 2,920,227 1/60 Dohler 333--31 20 2,932,7614/60 Epsztein 315--3.6 2,989,661 6/61 Cutler 3153.6

FOREIGN PATENTS 814,120 5/ 59 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner.

, ELI J. SAX, Examiner.

1. A SLOW WAVE STRUCTURE WHICH WILL PROPAGATE RADIO FREQUENCY WAVES OVERA FREQUENCY BAND WHOSE WIDTH EXCEEDS FOUR PERCENT OF THE MID-BANDFREQUENCY COMPRISING A PAIR OF PARALLEL PLANE CONDUCTIING SIDE WALLS ANDA GRATING CONNECTING THE TWO SIDE WALLS ALONG THE LENGTH OF THESTRUCTURE IN A COMMON PLANE PERPENDICULAR TO THE SIDE WALLS WITH THESIDE WALLS EXTENDING ABOVE AND BELOW THE PLANE OF THE GRATING, AND INWHICH THE GRATING IS COMPOSED OF CONTINUOUS INTERSECTING CONDUCTINGSTRANDS ARRANGED TO FORM A REGULAR PATTERN OF STAGGERED APERTURES, SAIDGRATING BEING OPERABLE IN BOTH AMPLIFIER AND BACKWARD WAVE TUBES.